A Review of the Ecomorphology of Pinnotherine Pea Crabs (Brachyura: Pinnotheridae), with an Updated List of Symbiont-Host Associations

Abstract

Almost all pea crab species in the subfamily Pinnotherinae (Decapoda: Brachyura: Pinnotheridae) are considered obligatory endo- or ectosymbionts, living in a mutualistic or parasitic relationship with a wide variety of invertebrate hosts, including bivalves, gastropods, echinoids, holothurians, and ascidians. While the subfamily is regarded as one of the most morphologically adapted groups of symbiotic crabs, the functionality of these adaptations in relation to their lifestyles has not been reviewed before. Available information on the ecomorphological adaptations of various pinnotherine crab species and their functionality was compiled in order to clarify their ecological diversity. These include the size, shape, and ornamentations of the carapace, the frontal appendages and mouthparts, the cheliped morphology, the ambulatory legs, and the reproductive anatomy and larval characters. The phylogenetic relevance of the adaptations is also reviewed and suggestions for future studies are made. Based on an updated list of all known pinnotherine symbiont–host associations and the available phylogenetic reconstructions, it is concluded that, due to convergent evolution, unrelated species with a similar host interaction might display the same morphological adaptations.

Crustaceans and Marine Heterobranchia: A New Symbiotic Relationship in the Mediterranean Sea

“…Considering that sacoglossans produce a lot of mucus and that in particular E. timida contains repellent and unpalatable polypropionate compounds [7], it is plausible to hypothesize that the crab here reported is the case of a symbiotic association between a Pinnotherinae species and the sacoglossan E. timida that was never reported before. In fact, the small size of the observed crab, up to 1 mm long, the hard and ornamented carapace, the presence of the eyes visible on the dorsal surface and its ability to mimic the red spot of E. timida are coherent with the typical characteristics showed by other Pinnotherinae species [4]. In particular, the hard carapace and its flattened shape could be a functional adaptation to the particular host it lives on.…”

“…In fact, the "pea crabs", as the members of this latter family are commonly named, comprise around 320 accepted species grouped in four subfamilies: Pinnotherinae, Pinnixinae, Pinnothereliinae, and Pinnixulalinae. While members of the three latter subfamilies are commensal symbionts living inside the holes and tubes of living annelid and sipunculid worms and inside mud shrimp burrows [3,4], members of Pinnotherinae are defined as small symbiotic crabs, living on different hosts, mainly on molluscs [4]. It is reasonable to attribute the small crab here reported to the Pinnotherinae.…”

“…In fact, the "pea crabs", as the members of this latter family are commonly named, comprise around 320 accepted species grouped in four subfamilies: Pinnotherinae, Pinnixinae, Pinnothereliinae, and Pinnixulalinae. While members of the three latter subfamilies are commensal symbionts living inside the holes and tubes of living annelid and sipunculid worms and inside mud shrimp burrows [3,4], members of The observation revealed the crustacean as a Decapoda Brachyura belonging to the Pinnotheridae family. In fact, the "pea crabs", as the members of this latter family are commonly named, comprise around 320 accepted species grouped in four subfamilies: Pinnotherinae, Pinnixinae, Pinnothereliinae, and Pinnixulalinae.…”

“…Unfortunately, it was not possible to assign the pea crab here investigated at the genus level, due to the limit of the instrument used to observed it, but there are some ecological features that could give interesting insights. The trophic strategy of Pinnotherinae crabs consist of eating the mucus produced by their hosts, which contains organic particles and nutrients [4]. Additionally, some ectosymbionts or species associated with spacious host (like Gastropoda and Echinoidea) or that frequently leave their hosts, show external morphological adaptations such as the presence of eyes sometimes visible through the dorsal surface, for example, Pinnaxodes bipunctatus (Nicolet in Gay, 1849) and Calyptraeotheres camposi (Ayón-Parente and Hendrickx, 2014), a hard carapace and an ornamented body shape.…”

“…Additionally, some ectosymbionts or species associated with spacious host (like Gastropoda and Echinoidea) or that frequently leave their hosts, show external morphological adaptations such as the presence of eyes sometimes visible through the dorsal surface, for example, Pinnaxodes bipunctatus (Nicolet in Gay, 1849) and Calyptraeotheres camposi (Ayón-Parente and Hendrickx, 2014), a hard carapace and an ornamented body shape. This last feature is functional, making the mucus adhere to the cracks of the carapace and favoring the anchorage, overlaying the external body to conceal the crab or making the crab less palatable [4].…”